We now analyze the results for the flow-induced birefringence measured with the high-speed polarization camera for an extending liquid polymer.
When all the polymer chains are aligned with the optical axis, the flow-induced birefringence \(\delta _{n}\)  due to the polymer chain morphology can be derived from the measured retardation \(\delta \)  [Eq. (REF )].
Figure REF  show plots of the birefringence \(\delta _{n}\)  vs normalized time \(\hat{t}\)  (normalized by the transition time from the IC to the EC regime).
Here, the normalized times within the EC regime for the PEO with molecular weights \(M_{w} = 1\times 10^6\)  and \(5\times 10^6\)  are from 1 to 1.7 and from 1 to 2.3, respectively.
These results confirm that the birefringence increases exponentially within the IC regime, has a constant value within the EC regime, and increases again within the VC regime.
Moreover, the plot of birefringence has an inflection point at the transition from the IC to the EC regime.
Within the EC regime, the variations of the birefringence with respect to the overall range are \(\pm 1.45\%\)  and \(\pm 7.98\%\)  for PEO with molecular weights \(M_{w} = 1\times 10^6\)  and \(5\times 10^6\) , respectively.
The fact that the value of birefringence remains constant within the EC regime is consistent with the theoretical basis of CaBER-DoS, according to which the uniaxial extensional stress is constant within the EC regime [1]}, [2]}, [3]}, [4]}, [5]}.
